KUWAIT, Nov 4 (Reuters) - The opposition in Kuwait will press ahead with a march on Sunday to protest against new voting rules despite the government warning it will not tolerate unsanctioned demonstrations, an activist said.

The OPEC member and U.S. ally said on Saturday it had not authorised any protests and the prime minister warned police could use force if the authorities felt the Gulf Arab state was under serious threat.

Kuwait has avoided the kind of mass pro-democracy unrest seen in other Arab countries, but tensions have mounted between the elected parliament and the government, which is dominated by the Al-Sabah ruling family.

The authorities outlawed gatherings of more than 20 people last month after an opposition-led demonstration by thousands ended in clashes between protesters and police in which at least 30 people were taken to hospital.

“We will continue. The opposition no longer cares about government statements,” an opposition activist said, declining to be named.

Kuwait’s stock index fell to its lowest level since July 2004 on Sunday, according to Reuters data.

Opposition politicians, youth groups and their followers have taken part in recent demonstrations protesting against changes to an electoral law, which was announced last month by ruler Sheikh Sabah al-Ahmad al-Sabah.

Some opposition politicians have said the changes are an attempt to give pro-government candidates an advantage in parliamentary elections set for Dec. 1, and have said they will boycott the poll.

“A COUP AGAINST THE CONSTITUTION”

The opposition has called the changes - which allow voters to choose only one candidate per electoral district - “a coup against the constitution”, saying the reform would prevent its candidates from winning the majority they won in the last vote.

Forging an electoral alliance, which depends on supporters of one candidate voting for another in exchange for reciprocal support, would become unfeasible under the new system, they say. Political parties are banned in Kuwait so lawmakers rely on being able to form blocs based on policy and family ties.

However, the government says the amendments to the electoral law were needed to preserve national unity.

Demonstrations about local issues occur frequently in Kuwait, which tolerates more public dissent than some of its Gulf neighbours. Violence had previously been very rare.

“The state does not ... want to use violence, but when the security of the homeland is endangered and the safety of its citizens at risk, it will not hesitate to use force in the framework of the law and the constitution,” state news agency KUNA quoted Prime Minister Sheikh Jaber al-Mubarak al-Sabah as saying late on Saturday.

An opposition bloc, made up of Islamist, liberal and tribal lawmakers, won a majority at the last elections in February. But that parliament was effectively dissolved by a court ruling in June which reinstated a more government-friendly assembly.

However, MPs boycotted that assembly, meaning it was unable to meet. The emir then dissolved it and called for snap elections after months of political deadlock.

Kuwait has one of the most open political systems in the Gulf and is home to an elected parliament with legislative powers. But the 83-year-old emir has the final say in state affairs and picks the prime minister, who in turn selects a cabinet.